1. Field of the Invention
The present invention relates to a buried heterostructure type semiconductor laser device and a method for producing the same.
2. Description of the Related Art
A conventional buried heterostructure type semiconductor laser device (hereinafter, referred to as xe2x80x9cBHLDxe2x80x9d) includes a light emission region, which is formed by an active layer stripe, and a buried layer for burying the active layer stripe, and separation grooves. A separation groove is provided on each side of the light emission region for electrical isolation in order to prevent capacitance of peripheral elements from affecting the light emission region. Outside the separation grooves, no active layer stripe is provided, and only the buried layer is formed on a substrate.
A method for producing the conventional BHLD will be briefly described below. First, semiconductor layers for forming an active layer stripe are crystal-grown on the substrate, and a dielectric film is formed thereon in a stripe shape. Then, the semiconductor layers are etched using the dielectric film as a mask to form the active layer stripe. Thereafter, the dielectric film is removed, and the active layer stripe is buried by using a liquid growth method. In this process, the buried layer causes difficulty in locating the active layer stripe with accuracy. Therefore, in order to perform subsequent production steps, a portion of the buried layer on the substrate is removed by etching to expose the active layer stripe. After the active layer stripe has been located, an alignment key is formed on the active layer stripe for use in the steps of forming a p-type electrode, forming separation grooves, gold-plating, forming scribe lanes, mounting, etc.
In this conventional method for producing the BHLD, for example, the alignment key used in each of the above-described steps may cause at least two deviations, i.e.: an alignment key position formed on the exposed active layer stripe may deviate from a predetermined position; and a pattern position which is formed in each step using the alignment key as a reference line may deviate from a predetermined position.
In the case of flip chip-mounting of a semiconductor laser device, a light emission point of the semiconductor laser device is aligned with an optical axis of an optical fiber combined with a chip carrier by combining an alignment key for mounting, which has been formed on a surface of the semiconductor laser device, with an alignment key formed on the chip carrier. However, as described above, since the alignment key for mounting formed on the surface of the semiconductor laser device may cause at least two deviations, in principle, alignment of the optical fiber with the light emission point after the flip chip-mounting is affected by such deviations. Furthermore, when a plurality of active layer stripes are provided, a deviation xcex8 is caused between an alignment key 602 and an active layer stripe 601 as illustrated in FIG. 7, and accordingly, alignment accuracy is further degraded.
In recent years, development has been eagerly carried out for the purpose of reducing production costs of a module including a semiconductor laser device mounted thereon. In order to realize such a module, simplified production/assembly steps achieving high light-coupling efficiency are required. Conventionally, to this end, the semiconductor laser device is flip chip-mounted on a module by passive alignment in which the semiconductor laser device is aligned with an optical fiber of the module using only their alignment keys.
However, in such a conventional method, the positional accuracy of the alignment key used for mounting with respect to the light emission point is poor. Therefore, even if the alignment key alignment for mounting with an alignment key on the chip carrier is perfectly performed, the light emission point is not necessarily accurately placed on the optical axis of the optical fiber. That is, the optical axis of the optical fiber is misaligned with that of the semiconductor laser device. Thus, it is difficult to achieve high light-coupling efficiency. Moreover, when removing a portion of the buried layer on the active layer stripe by etching, organic contamination may occur on a surface of the active layer stripe. Such contamination reduces production yield.
According to one aspect of the invention, there is provided a semiconductor laser device including a substrate, a light emission region provided on the substrate, and an alignment stripe provided on the substrate so as to be adjacent to the light emission region. The light emission region includes a first active layer stripe having a layered structure including a first waveguide layer, an active layer, and a second waveguide layer, a first buried layer formed so as to cover side faces of the active layer stripe, a second buried layer formed on the first buried layer, and a third buried layer formed on the second buried layer and the active layer stripe. The alignment stripe includes a second active layer stripe having a layered structure including the first waveguide layer, the active layer, and the second waveguide layer, and a selective growth mask formed on the second active layer stripe and formed of a material on which the first buried layer, the second buried layer and the third buried layer are incapable of growing.
According to one embodiment of the invention, at least one of faces of the third buried layer has a (111) face, a neighborhood face of the (111) face, a (001) face, and a neighborhood face of the (001) face which is present in the vicinity of both sides of selective growth mask in the alignment stripe.
According to another aspect of the invention, there is provided a semiconductor laser device including a substrate, a light emission region provided on the substrate, and an alignment region provided on the substrate so as to be adjacent to the light emission region. The light emission region includes an active layer stripe having a layered structure including a first waveguide layer, an active layer, and a second waveguide layer, a first buried layer formed so as to cover side faces of the active layer stripe, a second buried layer formed on the first buried layer, and a third buried layer formed on the second buried layer and the active layer stripe. The alignment region includes an active layer region having a layered structure including the first waveguide layer, the active layer, and the second waveguide layer, and a selective growth mask formed on the active layer region and formed of a material on which the first buried layer, the second buried layer and the third buried layer are incapable of growing.
According to one embodiment of the invention, at least one of faces of the third buried layer has a (111) face, a neighborhood face of the (111) face, a (001) face, and a neighborhood face of the (001) face which is present in the vicinity of the edges of selective growth mask in the alignment region.
According to another embodiment of the invention, the active layer stripe is formed so as to extend in a [011] direction, and the selective growth mask has at least one of the sides which includes a side which extends in a direction substantially parallel to the active layer stripe, a side which extends in a direction substantially perpendicular to the active layer stripe, and a side which extends in a direction crossing the active layer stripe at an angle of approximately 45 degrees.
According to still another aspect of the invention, there is provided a method for producing a semiconductor laser device including the steps of: growing a first semiconductor film which includes a first waveguide layer, an active layer, and a second waveguide layer on a substrate; forming on the first semiconductor film a selective growth film of a material on which second and third semiconductor films are incapable of growing; processing the selective growth film into two or more stripes; etching the first semiconductor film using the stripes of the selective growth film as masks, thereby forming an active layer stripe; growing the second semiconductor film as a buried layer on side faces of the active layer stripe and on a surface from which the first semiconductor film has been removed while the selective growth film is left unremoved; removing the selective growth film stripes while at least one of the two or more stripes is left unremoved; growing the third semiconductor film after the selective growth film has been removed.
According to one embodiment of the invention, the method for producing a semiconductor laser device further includes the step of forming an alignment key using an edge formed by a face of the third semiconductor film grown in a [100] direction and at least one of faces of the third semiconductor film includes: a (111) face; a neighborhood face of the (111) face; a (001) face; and a neighborhood face of the (001) face which is present in the vicinity of a side of the unremoved selective growth film, or using edges of the unremoved selective growth film as reference lines.
According to still another aspect of the invention, there is provided a method for producing a semiconductor laser device including the steps of: growing a first semiconductor film which includes a first waveguide layer, an active layer, and a second waveguide layer on a substrate; forming on the first semiconductor film a selective growth film of a material on which second and third semiconductor films are incapable of growing; processing the selective growth film into one or more stripes and into a prescribed form which has at least one of the sides including a side extending in a direction substantially parallel to the stripe of the selective growth film, a side extending in a direction substantially perpendicular to the stripe, and a side crossing the stripe form at an angle of approximately 45 degrees; etching the first semiconductor film using the stripe of the selective growth film as a mask, thereby forming an active layer stripe, and etching the first semiconductor film using the selective growth film processed into the prescribed form as a mask, thereby forming an alignment region; growing the second semiconductor film as a buried layer on side faces of the active layer stripe, side surfaces of the alignment region, and a surface from which the first semiconductor film has been removed while the selective growth film is left unremoved; removing the selective growth film except for the selective growth film processed into the prescribed forms; and growing the third semiconductor film after the selective growth film has been removed.
According to one embodiment of the invention, the method for producing a semiconductor laser device further includes the step of forming an alignment key using an edge formed by a face of the third semiconductor film grown in a [100] direction and at least one of faces of the third semiconductor film includes: a (111) face; a neighborhood face of the (111) face; a (001) face; and a neighborhood face of the (001) face which is present in the vicinity of both sides of the unremoved selective growth film, or using edges of the unremoved selective growth film as reference lines.
According to the above-described structure, since the active layer stripe and the selective growth mask are produced during the process of forming the layered structure, the light emission point and the alignment stripe or the alignment region can be formed as designed so as to have a predetermined positional relationship therebetween. Therefore, when the alignment keys for use in subsequent steps are formed by using edges of the selective growth mask as reference lines, a deviation of the alignment keys from the predetermined position with respect to the light emission point is reduced by half in comparison with that of the conventional structure. Accordingly, the misalignment of the alignment key for mounting formed on the semiconductor laser device surface with respect to the alignment key on the chip carrier of the chip carrier is also reduced by half. Moreover, it is not required to remove the buried layer on the active layer stripe by etching, as performed in the conventional method, in order to locate the active layer stripe. Thus, organic contamination on the surface of the light emission region can be prevented, whereby the light emission efficiency can be improved. In addition, the deviation xcex8 of the alignment key from a predetermined position with respect to the active layer stripe can be prevented.
Alternatively, in the case of flip chip-mounting a semiconductor laser device on a chip carrier, when the semiconductor laser device is aligned with the chip carrier using the edges of the alignment stripe or the alignment region as reference lines in place of the alignment keys for mounting, the mounting accuracy is significantly improved in comparison with that of the conventional structure.
The (111) or (001) faces are formed in an alignment stripe or an alignment region so that sharp edges are formed in a vertical cross section of the semiconductor laser device taken along a line which extends in any one of the following directions: a direction which is substantially parallel to a longitudinal direction of a resonator of the semiconductor laser device (stripe direction); a direction which is substantially perpendicular to the stripe direction; and a direction which crosses the stripe direction at an angle of 45 degrees. Therefore, such edges of the alignment stripe or the alignment region can be more readily identified. Thus, when the semiconductor laser device is mounted on the chip carrier using such edges as reference lines for alignment, alignment accuracy improves. According to the present invention, the (111) faces may be either a (111)A or (111)B face. When the buried layers are ideally grown, a (001) face may be formed besides a (111)A or (111)B face. Each of these faces may deviate by xc2x120 degrees from a prescribed angle according to a shape of a cross section of the stripe, or the width of the selective growth mask. Such a face is hereinafter referred to as a xe2x80x9cneighborhood facexe2x80x9d.
Thus, the invention described herein makes possible the advantages of providing: (1) a semiconductor laser device in which a location of an active layer stripe can be accurately identified to prevent the deviation of alignment keys from predetermined positions with respect to a light emission point, the production yield can be improved without causing surface organic contamination of a light emission region and the semiconductor laser device can be aligned with an optical fiber with high accuracy, whereby production cost of the semiconductor laser module can be reduced; and (2) a method for producing such a semiconductor laser device.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.